The interaction between transposed autonomic ganglion and the intact, still-developing brain of 6 to 14 day old rats have been examined by light and electron microscopy. When 1 cubic mm fragments of the superior cervical ganglion (SCG) are transplanted to the undamaged pial surface of the cerebellum in young rats, cells - both neuronal and glial - of the external granule layer (EGL) are arrested in the molecular layer. Three months after they should have migrated from the internal granule layer, whole laminae of these cells not only fail to migrate in the normal direction but some leave the cerebellum and, together with cerebellar neuropil, migrate into the transplant. Within the transplant, some of the ganglion cells die but there is a marked proliferation of axons. Unmyelinated axons are enclosed by Schwann cells and these, in turn, by fibroblasts. After six months, many axons have become myelinated. The formation of nodes of Ranvier indicates cooperation between Schwann cells which also bedeck ependymal and glial surfaces as well as neuronal processes.